[feat] zc.recipe.egg: Support environment in :develop

Currently only zc.recipe.egg:custom supports setting environment
variables, and zc.recipe.egg:develop does not.

My motivation for allowing setting environment in :develop is
wendelin.core

    https://lab.nexedi.cn/nexedi/slapos/blob/b5faab3b/component/wendelin.core/buildout.cfg

There we have [wendelin.core] part which installs released egg from
pypi, and [wendelin.core-dev] part which installs wendelin.core from
its latest git version via zc.recipe.egg:develop .

The problem is, wendelin.core for setup.py to work, needs git available,
and with slapos we usually don't have git available on base system, so
we build it by our own and do something like

    [wendelin.core-dev]
    recipe = zc.recipe.egg:develop
    environment = wendelin.core-dev-env

    [wendelin.core-dev-env]
    # wendelin.core-dev needs git to build
    PATH = ${git:location}/bin:%(PATH)s

and the problem is environment does not currently work for
zc.recipe.egg:develop, and thus git is not found -> build fails.

~~~~

In order to support environment in :develop, we just move environment
setting/restoring bits from Custom to Base, and provide Base.install() which
uses this bits. Custom & Develop .install() becomes ._install() which gets
hooked into Base.install() .

I've tested the patch only manually, because currently automated tests are
broken in a lot of places for slapos.buildout and zc.recipe.egg .

/cc @kazuhiko, @Tyagov

zc.recipe.egg_/src/zc/recipe/egg/custom.py

@@ -29,11 +29,47 @@ class Base:
 
         options['_d'] = buildout['buildout']['develop-eggs-directory']
 
+        environment_section = options.get('environment')
+        if environment_section:
+            self.environment = buildout[environment_section]
+        else:
+            self.environment = {}
+        environment_data = list(self.environment.items())
+        environment_data.sort()
+        options['_environment-data'] = repr(environment_data)
+
         self.build_ext = build_ext(buildout, options)
 
+    def install(self):
+        self._set_environment()
+        try:
+            return self._install()
+        finally:
+            self._restore_environment()
+
     def update(self):
         return self.install()
 
+    def _set_environment(self):
+        self._saved_environment = {}
+        for key, value in list(self.environment.items()):
+            if key in os.environ:
+                self._saved_environment[key] = os.environ[key]
+            # Interpolate value with variables from environment. Maybe there
+            # should be a general way of doing this in buildout with something
+            # like ${environ:foo}:
+            os.environ[key] = value % os.environ
+
+    def _restore_environment(self):
+        for key in self.environment:
+            if key in self._saved_environment:
+                os.environ[key] = self._saved_environment[key]
+            else:
+                try:
+                    del os.environ[key]
+                except KeyError:
+                    pass
+
 
 class Custom(Base):
 
@@ -54,23 +90,14 @@ class Custom(Base):
             options['index'] = index
         self.index = index
 
-        environment_section = options.get('environment')
-        if environment_section:
-            self.environment = buildout[environment_section]
-        else:
-            self.environment = {}
-        environment_data = list(self.environment.items())
-        environment_data.sort()
-        options['_environment-data'] = repr(environment_data)
-
         options['_e'] = buildout['buildout']['eggs-directory']
 
         if buildout['buildout'].get('offline') == 'true':
-            self.install = lambda: ()
+            self._install = lambda: ()
 
         self.newest = buildout['buildout'].get('newest') == 'true'
 
-    def install(self):
+    def _install(self):
         options = self.options
         distribution = options.get('egg')
         if distribution is None:
@@ -100,36 +127,11 @@ class Custom(Base):
             extra_path = os.pathsep.join(ws.entries)
             self.environment['PYTHONEXTRAPATH'] = os.environ['PYTHONEXTRAPATH'] = extra_path
 
-        self._set_environment()
-        try:
         return zc.buildout.easy_install.build(
             distribution, options['_d'], self.build_ext,
             self.links, self.index, sys.executable,
             [options['_e']], newest=self.newest,
             )
-        finally:
-            self._restore_environment()
-
-
-    def _set_environment(self):
-        self._saved_environment = {}
-        for key, value in list(self.environment.items()):
-            if key in os.environ:
-                self._saved_environment[key] = os.environ[key]
-            # Interpolate value with variables from environment. Maybe there
-            # should be a general way of doing this in buildout with something
-            # like ${environ:foo}:
-            os.environ[key] = value % os.environ
-
-    def _restore_environment(self):
-        for key in self.environment:
-            if key in self._saved_environment:
-                os.environ[key] = self._saved_environment[key]
-            else:
-                try:
-                    del os.environ[key]
-                except KeyError:
-                    pass
 
 
 class Develop(Base):
@@ -139,7 +141,7 @@ class Develop(Base):
         options['setup'] = os.path.join(buildout['buildout']['directory'],
                                         options['setup'])
 
-    def install(self):
+    def _install(self):
         options = self.options
         return zc.buildout.easy_install.develop(
             options['setup'], options['_d'], self.build_ext)

zc.recipe.egg_/src/zc/recipe/egg/custom.rst

@@ -503,6 +503,10 @@ swig-cpp
 swig-opts
    List of SWIG command line options
 
+environment
+   The name of a section with additional environment variables. The
+   environment variables are set before the egg is built.
+
 To illustrate this, we'll use a directory containing the extdemo
 example from the earlier section: